Process Description:
Paper Industry; Lime Mud & Sludge Recalcination

Paper Industry; Lime Mud & Sludge Recalcination

Molten smelt, a by-product of the Kraft pulping process for making paper, is dissolved in water to become green liquor. Clarified green liquor is then reacted with lime in a slaker (called causticizing). This produces a fresh pulping chemical cooking solution (white liquor) and lime mud or lime sludge. Lime mud or sludge is in turn recalcined to lime or calcium oxide (CaO) by removal of carbon dioxide with heat. The result is fresh lime, ready for use in the slaker.

The Enders' fluidized bed lime calciner is a two-stage fluidized bed reactor, in which the upper compartment is the dryer and the lower compartment the calciner. Lime mud is pumped into the drying stage where it is dried with hot gases flowing through the bed from the calcining stage and becomes part of the fluidized bed. Bed level is maintained since it is continually being skimmed off the top (overflows to feed transfer piping and into calciner stage).

Fuel oil is injected into the calcining compartment bed to provide the process heat. The low calcining temperature of approximately 1550oF results in excellent fuel economy, prevents over-burning of lime and produces a particularly reactive type of lime. Calcined lime pellets discharge the calcining via a variable speed metering screw conveyor, a cooling screw and then a bucket elevator to storage.

Combustion air required for oxidation of fuel is furnished as fluidizing air by the fluidizing air multistage centrifugal blower. The fluidizing air is preheated in an oil or gass fired air preheater for start-up to about 1000oF. Air discharges the blower at about 160oF because the heat of compression going through the blower. Combustion gases leaving the calcining compartment are stripped of their dust loading in a cyclone before serving as fluidizing gas and heat in the drying stage Dust from the calcining and drying stage cyclones is returned to it's respective fluid bed.

A two stage scrubber system is provided for particulate removal; first a venturi followed by a packed tower. Make-up is added to the last stage where excess water overflows to the venturi stage from which a bleed stream is pumped out to the clarifier.

Other inorganic chemicals contained in the lime mud will circulate with the recalcined lime. Sodium salts which may precipitate in the color removal system should solubilize again and leave in this totally oxidized state. There is a practical limitation imposed upon the sodium content of calcined lime. The sodium content as Na2O must be below 8 percent, based on calcined product weight in order to insure complete control over the rate of agglomeration in the calcining stage.

Start-up and Operation
The lime recalcining system can be operated by one or two people, depending upon the sophistication provided by the control loops. Generally, one person should suffice when the calciner is installed adjacent to related facilities. Start-up and shut-down is accomplished manually, however provisions are made for an automatic shut-down sequence in the event of a major failure. Typical start-up time is 12 to 16 hours from a completely cold system; 4 to 6 hours from a weekend shut down.